1. Field of the Invention
This invention relates to welding and, more particularly, the present invention is directed to an improved welding torch which utilizes a preheating step whereby the wire heat content is increased by means other than welding current.
2. Description of the Prior Art
In a conventional gas-metal arc welding (GMAW) process, heating of the alloyed wire prior to deposition is accomplished by passing welding current through a certain wire length, commonly referred to as the wire stick-out. The power consumed in heating this wire is equal to the product of the square of the welding current and the resistance of the wire. In order to increase the wire deposition rate, the heat content of the wire is increased by increasing the electrical stick-out or the welding current or both.
Excess wire stick-out leads to uncontrolled wire wandering and/or poor deposit quality. Therefore, welding current is the predominant variable that determines deposition rate and the mode of metal transfer through the arc. However, current intensification leads to more power dissipation in the arc. Since approximately 65 to 85% of the arc heat is conducted into the metal workpiece, a higher current would increase arc penetration while increasing deposition rate and decreasing dilution. Although dilution can be reduced by employing higher welding current and slower speeds of travel, these means of control have practical limitations. Higher heat input per unit of length can generate excessive assembly distortion and metallurgical damage in both the deposit and the base metal workpiece, such as heat affected zones (HAZ) underbead cracking and hot-cracking.
In the conventional hot wire gas-tungsten arc welding (HWGTAW) process, heating of the alloyed wire prior to deposition is accomplished by passing heating current through a certain length of wire stick-out. As in the GMAW process, higher wire heat content is adjusted by increasing wire stick-out or increasing heating current or both. This makes the HWGTAW process subject to the similar difficulties experienced by the GMAW process. My issued U.S. Pat. No. 4,447,703 which is assigned to the assignee of the present invention and incorporated by reference herein, teaches a technique for overcoming the aforedescribed difficulties. The aforesaid patent describes a technique for preheating the wire at a location remote from the arc and molten pool, thereby reducing the required welding current and wire stick-out length. Thus, in a gas metal arc welding method in which a power supply feeds welding current through a welding wire electrode into a metal workpiece, the aforesaid patent adds a preheating step in which the wire heat content is increased by means other than the welding current. By increasing the heat content of the wire, a significant reduction in weld dilution is achieved. This preheating is accomplished by passing a current through a wire segment prior to the wire's entry into the stick-out region. The use of preheating wire limits the length of wire stick-out required to reach a desired wire temperature for a given current in the wire, thereby minimizing wire wandering, improving deposit quality and minimizing arc-wire interaction.
My U.S. Pat. No. 4,590,358 which is assigned to the assignee of the present invention and which is incorporated herein by reference, describes a concept for electrically isolated hot wire operations that maximizes the electric contact consistency between the consumable electrode and the electric contact tips. A consumable electrode passes through the bores in two electrode contact tips which are separated by a dielectric member having a bore therethrough. The aforesaid application discloses that the key to obtaining consistent preheating of the wire by passing current between the two electrode contact tips and the length of consumable electrode therebetween resides in ensuring that the bores through which the consumable electrode passes are never both coaxial with the torch body and each other.
It is an object of this invention to provide an improved welding torch which incorporates the principles disclosed by my earlier issued patent and application.
It is also an object of this invention to provide a welding torch which permits consistent preheating of the consumable electrode by passing current through a pair of electrode contact tips separated by a dielectric member.
It is yet another object of this invention to provide a welding torch design which requires minimum space requirements and is ideally suited for use in automated applications.